Ultrahigh-frequency bridge circuit and apparatus



Jan. 12, 1954 w. BARROW ULTRAHIGH-FREQUENCY BRIDGE CIRCUIT AND APPARATUS 5 Sheets-Sheet 1 Filed Jan. 25, 1947 MHQ HUM:

Jan. 12, 1954 L BAIQRQW 32 ULTRAHIGH-FREQUENCY BRIDGE CIRCUIT AND APPARATUS Filed Jan. 25, 1947 5 Sheets-Sheet 2 v INVENTOR.

W/L MD? A. B/QRROW ATTORNEY Jan. 12, 1954 w. BARROW 2,556,132

ULTRAHIGH-FREQUENCY BRIDGE CIRCUIT AND APPARATUS Filed Jan. 25, 1947 5 Sheets-Sheet 3 IN VEN TOR. VV/L Mil? A .B/IR/m w Jan. 12, 1954 W. L. BARROW ULTRAHIGH-FREQUENCY BRIDGE CIRCUIT AND APPARATUS Filed Jan. 25, 1947 5 Sheets-Sheet 4 WEST INVENTOR. VV/L MEA L. BHRAO w Jan. 12, 1954 W. L. BARROW ULTRAHIGH-FREQUENCY BRIDGE CIRCUIT AND APPARATUS Filed Jan. 25, 1947 5 Sheets-Sheet 5 SOURCE gg M /37 e E6 37 3 INVENTOR. W/l. M157? L BHRRO W Mandi/97b? A ,4 TTOR/VEY ky/ w .a brid whilei th fiorm'adanted' for makliig s e ma etic g s w i h pic mafi hybrid coils.

Patented Jan. 12, 1954 UNITED STATES PATENT 05mg];

ULTRAHIGH-FREQUENCY BRIDGE-01.89111! 4N1.) e eres Wilmer L. liarrcw, Manhass et, N. Y. Annl c fien nu 2 .47, etiel N9: 725- 13 Claims. (01.

This invention relates to ultra-high-frequency bridge circuits and refers, more partic'u'larly, to bridge circuits in which transmission 1 es are essential elements. The present applicationTis a continuatiomin-part of my application serial No. 37,6,253 filed January 28, 1941, not; Patent No. 2,416,790 granted March 4, 1947.

The difficulties encountered in extending the use of conventional bridge circuits lid ultra high frequencies are well known and are very great. Bridges have been constructed that function at ireque'ncies as high as ten mega'cyeles'oi' even somewhat higher but little work has been done at yery high frequencies such as those of'the order of 190 megacycles'and above, duets coin- 3 plications and uncertainties which arise and which increase rapidly with increasing frequency. For example, as applied to the measurement of impedance, the use of bridge circuits at ultra high frequencies is adversely affected by mean.

pedance and" calibration of the indicating instrument, by the ,eiiect of leadsto auxiliary 1l1ipmerit, and by similar factors which ,vary with freguency, so that the useful frequency range of a ,given piece of equipment is usually very limited. In addition, conventional circuits do not provide ,true equivalents of .;bridge circuits at high irequenciesin many ,apnlications where it "dvantage of the char ctei' is' desirable to ,takev ist s et /b d e con i Y M impro e l w fi making se f t smisr i line a d e e n am a vantages, pr des l e mplete equiyal m edanc me su emen s. the impedanq vo t e ndic t n inst umen tand t ca b a i n d ea enter into theprecision .of the measurement;

The te tr nsmiss o l ne a sed 11' n in ts na rowe .se s er t an 'm an if r s'tricte d' path having distributed electrical ,con; stants, such as cf the two-wire or hollow Wave guide or dielectric guide type, but it also has a *wider meaning which includes; "in "addition, lumped impedance artificial transmission lines having propagation characteristics similar to those of actual transmission lines. While the length of transmission lines having distributed .constants is a .bar totheir .use in" mo'st"low fr'e quency apparatus, at high frequencies .they b e come relatively short, which makes their use practical in a wide:variety of applicaticns. gFo'r examp'1e, at:30 0 megacycles a quarter wave length line is approximately only 1 0 inches lengthf An object of theipresent invention'isto proseries-connected load for coaxial line bridges.

Another object is to provide novel arrangements for interconnecting a plurality of circuits with one'er' b o he ci cu 80 e in ibit interaction amongthe said plurality of "circuits; Anothe obiie t t p ide a b id i c t Q the character described employing metalpip'c wave guides and adapted for interconnection in systems of metal pipe'wave-g'uide construction.

A still further object is to provide a' trans: mis ioln l ne r dg cir which d 119W"- quire line transpositions and is therefore pa'rticu la y adaptedto utilize coaxial cable and"wave guideline St l no h r je t of e re en in en i n s to" pm me m bved s em i' in l.e bridge'circuits discussed above;

Qther objects and advantages of this invention will'becoineapparent as the description proceeds.

In the drawings,

Fig. 1 is a generalized schematic circuit diagram illustrating the connection of fountransmission lines in a bridge arrangemen wi h foul lumped-impedance lci rcl il elements connected at bridge points. k

Fig. 21s a similar schematic circuit diagram of the transmission linebr idge circuito ffig'. I having a high frequency source and an indicator as the circuit elements connected at a pair of opposite bridge points.

Fig. 3 is a similar generalized diagram of a bridge circuit having shunt-connected imped ances at a pair of opposite bridge points 'anda transpositiontin one of the lines.

Fig. .4 illustrates a similar generalized circuit diagram of a bridge having a shunt-connected source and a series-connected indicator.

.Fig. .5 illustrates a similar generalized circuit diagram of a bridge having a series connected source and shunt-connected indicator.

Figs. 16 to .Q'illustrate various forms of impedance elements suitable for connection at :bridge points. Fig. 10 is a schematic diagram partly in crosssectional view'of a shunt-connected impedance element for coaxial line bridges.

Fig. 11' isa similar schematic diagram of a variable impedance element for coa iiial line systems. V 'l ig; '12 is a similar schematic diagram pi ,e. shunt-connected source for coaxial line brid es Fig. i3 is ,a similar schematic diagram Fig. 14 is a schematic diagram or iaico'axial linelbridge similar'to Fig. 4.

' l5 .isflafscheinatic diagram .of a coa rial line tlQQilSQOSltiOn.

Fig. lfifisfa schematic diagram ,of a waveguide bridge s iriiilarto Fig. ,3.

Fig. 17 is a sche' atic diagram of a waveguide dm ra manag me Fig. 18 is a schematic diagram of a series-connected indicator for wave-guide bridges.

Fig. 19 is a generalized schematic circuit diagram of a duplex radio communication system utilizing a transmission line bridge.

Fig. 20 is a similar generalized diagram of a doubly balanced bridge using two sources.

Figs. 21 to 23 are generalized diagrams of bridges used in high frequency two-way repeaters.

Fig. 24 is a generalized diagram of a carriersuppression modulator using a transmission line bridge. I

Fig. 25 is a generalized diagram of a double antenna radio receiver or transmitter using a transmission line bridge.

Fig. 26 is a similar diagram of a modified form of a transmission line bridge circuit modulator.

Fig. 27 is a similar diagram of a bridge circuit utilizing artificial lines having lumped impedance circuit elements.

In Fig. 1 there is shown the basic diagram of a generalized bridge circuit composed of four transmission lines I, 2, 3, 4 arranged in tandem to form a closed loop, with lumped-impedance circuit elements Z1, Z2, Z3, Z4 connected respectively at the bridge points formed by the junctions between a pair of adjacent lines. In this figure, and in all the following figures, this representation of transmission line is not intended to indicate structurally any particular type of such line, but is intended as a general representation of any type of transmission line, the following being cited as examples and not by way of limitation:

Two conductors unshielded.

Two conductors shielded.

One conductor shielded (coaxial line). Hollow-pipe wave guide. Dielectric-wire wave guide.

For description of the last two types of transmission lines, reference may be had to U. S. Patents Nos. 2,129,711 and 2,12Q,'I12 issued to G. C. Southworth, wherein wave guides are defined. It will be shown below how these various types of line are constructed and connected to their corresponding circuit elements. The bridge point impedance elements Z1, Z2, Z3 and Z4 represent any type of circuit element or combination of circuit elements ofiering impedance of the twoterminal or four-terminal type, and may be the impedances of apparatus units, for example, generators or meters. Specific examples of impedance elements useful as Z1, Z2, Z3, or Z4 are described below and shown in subsequent figures.

In Fig. 2 there is shown a modification of Fig. l in which the impedance element Z1 is formed as a high frequency source or generator I connected in series with an impedance element Z1, which may be provided by the internal impedance of the source I. Similarly, impedance element Z3 is formed as an indicator 3 connected in series with an impedance Z3 which may be indicator impedance.

Ignoring for the moment the two impedance elements 22 and Z4. it may be considered that the four generalized transmission lines I, 2, 3 and 4 form a pair or" separate transmission paths I, 2 and 4, 3 between high-frequency source I and indicator 3'. If the source I impresses a high frequency voltage E1 across lines I and 4, electromagnetic waves will travel along both paths and impress two alternating voltages across the impedance Z3 of the indicator 3. Assuming that the lines have similar transmission characteristics and lengths, the two alternating voltages impressed by the two respective waves across impedance Z3 will have the same amplitude and phase while the current flowing in lines 2 and 3 and arriving at the terminals a, a by the two paths I, 2 and 4, 3 will have the same magnitude but opposite direction, resulting in a voltage antinode and a current node at this point.

If, as shown in Fig. 3, some means is included in one of the two transmission paths l, 2 and 4, 3 for shifting the phase of the transmitted wave by 180 (such as, for example, the transposition T shown in line i of Fig. 3 when using two-wire lines) the respective voltages at element Z3 resulting from transmission of waves from source I over the two paths I, 2 and 4, 3 will be of opposite phase, while the currents in lines 2 and 3 arriving at the terminals a, a will be in the same direction, resulting in a voltage node and at current anti-node at the element Z3 and indicator 3'. Other phase-shifters may be used in place of transposition T, such as a network which produces a phase shift of This may take the form of a transmission line having a length equivalent to an odd integral number of half wave-lengths, or the network may be composed of lumped impedances, preferably non-attenuating. The shifting of phase by the introduction of a length of transmission line is particularly suited to hollow-pipe wave-guide construction. The net voltage impressed across the indicator 3 will therefore be zero, and the circuit is balanced in the sense that an ordinary bridge network is balanced, since an input voltage applied to one pair of terminals is incapable of producing any output voltage across a second or conjugate pair of terminals. In referring to this condition, one bridge point of a transmission line bridge will be said to be balanced against another bridge point.

The insertion of equal impedances Z2 and Z4, respectively, at corresponding points in the two paths I, 2 and 4, 3 of either Fig. 2 or Fig. 3 changes, the magnitude of the voltages and currents at element Z3 and indicator 3 but not their relative phase or the equality of the magnitudes of these quanties. If the lines I, 2 3, 4 have dissimilar propagation characteristics, Z2 and Z4 may be made unequal to compensate for such differences, so as to maintain either current or voltage cancellation as in Fig. 2 or Fig. 3. It is not necessary that the length of path 2, 3 be equal to that of path I, 4, so long as the equality of voltages and currents is maintained at the indicator 3.

In place of the transposition T of Fig. 3 (or any other 180 phase shifter used in its place), the same ultimate result of bridge balance is obtained by the circuit of Fig. 4, where indicator 3' (which may still have an internal impedance Z3) is connected in series between adjacent lines 2 and 3, while the remainder of Fig. 4 is the same as in Fig. 2, source I being shunt-connected. By a series connection is meant that the indicator 3 is so connected between its adjacent lines 2 and 3 that electromagnetic wave energy flowing from one line toward the other is compelled to flow first through the indicator. This is to be distinguished from a shunt connection, where such energy divides into two parallel paths, rather than flowing sequentially from one element to another, as in a series connection.

As discussed above relative to Fig. 2, the currents arriving at the position of indicator 3 over the two paths I, 2 and 4, 3 are or equal magnitude and opposite direction, producing a current node. Since indicator 3 as connected in Fig. 4 is responsive to these currents, it will read zero upon balance of the two paths l, 2 and 4, 3 so that, with this connection, indicator 3' is conjugate to source l and a true bridge arrangement is obtained here also.

A balanced bridge arrangement can also be obtained by use of a series-connected source and shunt-connected load or indicator, as in Fig. '5. Source I, being connected across the series circuit formed by the open end of transmission line I and the open end of line 4 (each being a fourterminal network with distributed inductance and capacitance) excites lines I and 4 with voltages of the same magnitude but opposite phase. With paths I, 2 and 4, 3 balanced, these waves produce a voltage node at the terminals a, a at which indicator 3 is shunt-connected, again providing a true bridge arrangement. This construction of Fig. 5 is especially of advantage in certain types of two-wire line construction, since connection is made to one conductor only of a line or pair of adjacent lines. I

Where, as is usual, all circuit elements are linear and bilateral, the generator or source and the indicator (or any receiver or load taking its place) are interchangeable, either may be connected in series between terminal portions of two adjacent transmission lines or as a common shunt element to both.

As described above, the circuits of Figs. 3, 4 and 5 are true bridge circuits producing null outputacross the indicator 3 (or any load substituted for it) when an input voltage is applied by source I. It will be seen that, when source I and indicator 3' are similarly connected (both shunt-connected or both series-connected, as in Figs. 2 and 3) a 180 phase shifter or transposition in one of the two paths l, 2 or 4, 3 is necessary, these paths being otherwise electrically the same. However, when source l' and indicator or load 3 are oppositely connected (one shunt-connected and the other series-connected) nophase shifter or transposition is necessary.

Where, instead of being used for null indication, indicator 3 is intended to indicate the magnitude of the voltage or current anti-nodes at its position, the type of connection of either source If orindicator 3 in Figs. 3, 4 or 5 is reversed (that is, changed from series to shunt connection, or vice versa) or else a 180 phase shift or transposition is added in one of the transmission paths, I, 2 or 4, 3..

As stated above, the representation of transmission lines I, 2, 3,, and 4 in the drawings is intended to be generalized, and not to indicate any particular type of transmission line. However, where these transmission lines are of the parallelwire type (either shielded or unshielded) the two drawn lines of the figures (such as la, lb, for example) may. be considered to represent respectively the, two conductors of the parallel-wire transmission line. Similarly, where a coaxial or concentric transmission line is intended, one of the drawn lines (such as la). maybe considered to represent the inner conductor while the other (such as lb) represents the outer conductor, as will be more fully described below. Also, where dielectric-wire or hollow-pipe wave guides are intended, the drawn lines la, lb may beconsidered to-represent, in axial. cross-sectional. view, the outer opposed boundaries of the wave guide between whichexists the maximum voltage (which may be taken as the line integral of the electric field intensity). In the case of dielectric-wire guides, these drawn lines would represent the outer surface of the dielectric wire. In the case of hollow-pipe guides, these drawn lines would represent cross-sections of the pipe walls. Suchstructures are also described below.

As indicated above, impedance elements Z2 and Z4 may be formed in a variety of ways. Figs. 6 to '9 show various forms of such impedance elements suitable for use with two-wire transmission lines. Fig. 6 shows a single shuntconnected impedance element Zp connected between the two conductors la, ib, forming line I and connected respectively to conductors 2a, 2b forming line 2. Fig. '7 shows a simple seriesconnected impedance element 25 connected in series between conductors la, M. (It may alternatively be connected in series between conductors lb, 2b.) Also series elements may be used in both conductors la, 2a and lb, 2b. Fig.8 shows a T connection having shunt element Z and symmetrical series elements ZS while Fig. 9 shows 9. 1r connection having series elements Z5 and symmetrical shunt elements 21). Each type of connection has advantages in specific circuits. It is to be understood that any of these impedance elements may itself be a complex impedance or network made up of resistive and reactive components.

Fig. 10 shows one form which the balancing impedance Z: or Z4 may assume when using coaxial lines. Here shunt impedance element 'Z is connected at one end to the junction of inner conductors la, lb and at the other end to extension I92 of the junction of the outer conductors Sb, 2b, which extension also serves to maintain continuity of the shielding enclosure provided by the outer conductors.

An adjustable form of impedance element highly suitable for coaxial-type bridges is shown in Fig. 11. In this figure a section of coaxial line 53 is shown adapted to be joined at end 5| to a coaxial bridge. For example, the inner conductor of the device of Fig. 11 may be connected at its end 5| to the junction between conductors la, 2a of Fig. 10, while the outer conductor of the line section is connected to the junction of line outer conductors lb, 2b of Fig. 10. The adjustable coaxial impedance element of Fig. 11 comprises an adjustable shunt reactance in the form of a coaxial line section having outer conductor 54 and inner conductor 52- connected in shunt across the line 59. This section of line is provided with an adjustable short-circuiting plunger 55' so that its length D3 may be. varied. By this means, a reactanc-e of either positive or negative character and of any desired magnitude may be connected across the line. A further element of this adjustable impedance element comprises a line section having an outer conductor 58 and an inner conductor 59. Thislatter section. is connected to the line 56 at 60,, and this junction may be made adjustable by providing sliding connections between the two lines. Two adjustable plungers 6i and 62 provide any desired distance D4 between their respective faces. Of particular importance in its application to balancing a bridge is a length where, n; is a: positive integer; preferably 1, and is the; wavelength. For this length, the line section between the plungers Si and 62 provides a resonant electrical system offering purely resistive impedance, and this resonant system is connected to the transmission line at a distance D from one plunger. The magnitude of the resistive impedance connected across the line 50 at the point 89 may be varied between wide limits by appropriate adjustment of the length D5, as by ganged variation of plungers SI and 62 while maintaining fixed separation therebetween. By relatively varying the shunt reactance element and the substantially resistive element 58 any value of complex impedance may be made to appear at the terminal 5!, thereby providing an adjustable impedance element useful as elements Z: or Zr in a coaxial line bridge of the present type.

Figs. 12 and 13 illustrate how a shunt or series connection of source i or indicator 3' may be made for coaxial lines. Fig. .12 shows a shunt connection for source i, similar to the shunt connection shown in Fig. 10, impedance Z1 being connected in series with source 1 between the junction of inner conductors lb, 42) of lines I and 4 and the enclosing housing [8i connected to and forming a continuation of outer conductors lb, 42) of these lines.

Fig. 13 shows a series connection for source 3, in which source 3 and impedance Z3 are connected in series between inner conductors 2a, 3a of lines 2 and 3. Outer conductors 2b, 3b of these lines 2, 3 are extended as an enclosing shield E63 around inn; 3 and impedance Z3.

It will be understood that the arrangement of Fig. 12 may be used equally well for a shuntconnected indicator or load, merely substituting indicator 3 for source 1', and its impedance Z3 (if any) for impedance Z1. Similarly, in Fig. 13 source I and its impedance Z1 may be substituted for indicator 3 and impedance Z3 where a series-connected source is desired.

To illustrate how the elements of Figs. 10, 12 and 13 may be incorporated in a complete bridge system, reference is had to Fig. 14, which is a coaxial line form of the general bridge shown in Fig. in Fig. 14, the source and indicator of Figs. 12 and 13 are used, while the balancing impedances are as in Fig. 10. This construction eliminates the necessity of transpositions or phase-shifting devices in one of the lines. A schematic she- 11g of a transposition in a coaxial line, which illustrates the disadvantages of such arrangements, is found in Fig. 15. The transmission line consists of central conductor '1 and cylindrical metallic envelope or shield S coaxial therewith, the two conducting members being spaced at intervals by insulating washers Q. In order to connect the central conductor on one side oi the transposition to the shield on the opposite side and the shield on the first side to the central conductor on the other side to effect a transposition, a break in the otherwise continuous outer envelope is necessary which results in leaving a certain portion of the line unshielded and therefore subject to radiation losses. A further disadvantage of a transposition in acoaxial line is that currents may leave the internal shielded portion of the line at the place of transposition and travel along the outside of the shield, resulting in a substantial loss of the shielding cfiect of the outer conductor of the line. The basic series connection of Fig. 13 or 14, however, obviates this diiiiculty completely and provides a bridge which may be substantially perfectly shielded from interference by or to external systems.

To illustrate the structure of a wave guide form of bridge, attention is directed to Fig. 16, showing a hollow-pipe wave guide bridge of the general type shown in Fig. 3. In this figure the pipes are assumed to be seen in axial section, and it is to be understood that the two sectional lines connecting adjacent bridge points represent diametrically opposite portions of a single hollow conductor, preferably cylindrical in form, with any desired type of transverse cross-section. This way of realizing the bridge of the present invention afiords unusual adaptability to microwaves, i. e., waves whose lengths are, say, a few centimeters or less. At these extremely short wavelengths, ordinary lines, including even the coaxial form, have high losses because oi the imperfections in the insulators separating the two or more conductors of the line, and the construction of coaxial lines is diiiicult because their permissible size becomes quite small. The hollow-pipe modification, however, circumvents these difficulties and provides an enicient bridge even at the highest frequencies used in radio. Furthermore, the operation of a hollow-pipe bridge of this type with waves of the hollow-pipe type affords opportunities for a unique mode of operation. For example, the phase velocity of the waves may be made appreciably greater than the velocity of light in medium having the same constants as those of the medium inside of the pipe, permitting unexpected design features, such as a physical lengthening of the paths l, 2, 3, 4 for an effect equivalent to that of a coaxial line of shorter length, where this is desirable.

In the arrangement of Fig. 16, one terminal of indicator 3 is connected to an extension I03 of the cylindrical conductors or pipes 2, 3 forming the outer conducting boundary for the electromagnetic waves, while the opposite terminal is connected to a terminal rod or probe electrode l5 positioned to receive energy from the waves existing at the center of the pipes 2, 3. This arrangement is a shunt connection for the indicator 3, as shown for example, in Fig. 2. A similar connection is used for source I, with a probe Ii. A phase-shifting device, which may be a line an odd number of half wavelengths long, is shown at [5. The balancing impedances Z2 and Z4 intermediate the source I and indicator 3 in this construction take the form of pipe extensions 15 and ii, respectively, which may be terminated in any suitable manner, one form of such termination being shown in Fig. 1?. Appropriate terminations for hollow wave guides may be designed to give any desired impedance characteristic. It will be understood that the pipes l6 and i? may extend appreciable distances before reaching the point or termination.

It will also be understood that in any of the present bridges, the invention is not limited to the use of non-radiating purely absorptive impedance Z2 or Z4, as one or both of impedances Z2 or Z4 may be arranged for radiation or to include radiating means when it is desired to radiate part of the energy delivered to its bridge point by source i. in particular, with wave guide bridges, if the end or a wave guide is left open either with or without a flare, radiation will take place into the space beyond the mouth of the guide, as explained in my articles in the proceedings of the Institute of Radio Engineers, vol. 24, pages 1298, 1326 (October, 1936) and vol. 26, page 1498 (December, 1938). Such a radiating: wave; guide. may thus be coupledto; the; pipes [6 or i! of Fig. 16.. When. energy is radiated at abridge point adjacent. the source, an indicator at the point opposite the source may not be required. In this case indicator 3" may be replaced by a balancing impedance.

Anadjustable termin: ion for a transmission line bridge constructedv in hollow-pipe form according to the invention is shown in Fig. 17. In this figure, a section of hollow-pipe guide 40 is adapted to be connected at one end Al to a pipe extension of the bridge, for example, pipe [6 or H of Fig. 16-. I'he termination includes a conducting rod 42 extending; transversely to the pipe axis having a resistance 43 connected between. one endof the rod and an extension 3.!) of. the wall. of the pipe 40, while at its other end the rod 52 forms a part of a coaxial line section 44 having rod 42 as central conductor and also having an: outer conductor or shield 44 similar: to extension 39. Short-circui-ting plunger 45, which is shown as adjustable by means of the rods and handle 46, determines the efiecti'velength of the coaxial line section M. This coaxial line portion 44 of the terminal device of Fig. 17 provides a series reactance for the rod 42. A second plunger 41 tightly closes the end of the pipe 45 opposite to end 4|. The adjustable length of hollow pipe between the rod 42 and the plunger 4'? provides any adjustable shunt reactance for the transverse rod 42-; For a given value of the resistance or impedance 43 the lengths D1 and D2. may be so adjusted as to provide an impedance match for the pipe connected to 4|. Other adjustments of the lengths, D1 and D2. and other values of the impedance 53 provide impedances of, other desired values. Terminal meanssuch as illustrated in. Fig. 1'7 are particularly adapted for the operation of thebridge in hollow-pipe form with waves of the type referred to in the aforementioned Patent No. 2,129,712, as a symmetric magnetic or H1 type. Other terminal arrangements may be employed for this wave or other types of waves.

A series connection for a source or indicator when using hollow-pipe Wave guide is shown in Fig. 18. The indicator 3" in this. case'is connected to a pair of electrodes l8 and I9 positioned to receive energy from the. Waves arriving' over the two hollow-pipe lines 2' and 3, respectively. Indicator 3" is preferably enclosed in a shielded compartment 2|. Each pipe 2, 3 may be terminated in its characteristic impedance, this being represented symbolically only by impedances 20 and 20", to prevent undesired wave reflections. The actual terminations employed are to be understood as being of any conventional type suitable for connection to hollowpipe lines. If desired, source I" may be substituted: for indicator 3. to provide aseries-connected source. Said series-connected sources or indicators. maybe utilized in any of the bridge circuits shown in the preceding figures, when using. hollow-pipe, guides.

It. will be. understood that in any of the; present circuits, anyclesired: power-utilizing device or load may be used in place. of the indicator 3..

The preceding. discussion has described the construction. of: various; types of bridge circuits according to the present. invention,.-and has explained' the theory underlying their bridge-type apply: to any of the. forms of bridges described above; except. where otherwise noted.

Any of the above. typesof bridge producing, null output may be used for impedance measuring purposes. Thus, one of the balancing impedances (for example, Z2) may represent a circuit element of unknown impedance. The other balancing impedance (Z4) is then a calibrated impedance element.

Indicator 3' furnishes zero indication when the impedances Z2 and Z4 are equal, it being understood: that in this case the two transmission line paths I, 2. and 4, 3 connecting. source I and indicator 3" also: have the same transmission characteristics. Thus, the value of the unknown impeda-nce Z2 is determined from the known value of calibrated impedance Z4. The impedance of neither source. of indicator affects this relationship. Of particular importance is the fact that. the meter may have a low impedance and. that it requires no calibration either for linearity or voltage. Furthermore, the balance may be made. at any frequency provided only that the lines are long enough to function as transmission lines at that frequency.

Fig. 19 shows the system, of Fig. 4 applied to du-- plex communication. It will be noted that, in the preceding arrangements, While a balance is obtained between source I and indicator 3', power from source I is delivered to impedances Z2 and Z4. Also, if power is supplied directly to impedance Z2, it will be transmitted to indicator 3 This feature is utilized in Fig. 19. In this figure, generator I, which may be the output stage of a radio transmitter as before, supplies an E. M. F; E1 through its internal impedance Z1 by shunt connection to lines I and 4. Impedance element Zz in this case is an antenna 6" which simultaneously transmits and receives: en'- ergy while element Z4 is an adjustable balancing impedance therefor. Ifseries-connected element: Z is a radiov receiver at the bridge point opposite the transmitter l.', which is to be insensitive to the output of generator or transmitter I, and if element Z4 is adjusted to balance antenna impedance Z2, a network: is obtained in which power from the transmitter I is delivered to antenna 6' without affecting receiver 6, while power received by antenna 6' is. delivered to receiver 6.

Fig. 19 may also be used as an object detecting system. High frequency energy produced by transmitter I" is radiated by antenna 6 toward an object to be detected. Energy reflected by the object returns to antenna 6 and actuates receiver 6 to indicate the presence of the object.

The velocity of the object may be indicated by making the bridge circuit slightly unbalanced, so. that a small portion of the transmitter energy reaches the receiver together with the received energy. As is known, if the object is moving toward or away from the antenna, the energy reflected by it and received by antenna 6' and receiver 6 is shifted in frequency by the Doppler eflect. The received Wave and the small portion of the transmitted wave are heterod-ynedby receiver 6 and the resultant difference frequency is proportional to object velocity and may be suitably indicated.

Fig. 19 is thus an example of a circuit in which power is supplied both to impedance element Z1 (from source I) and to impedance element Z2 (from antenna 6'). Other circuits to be described herein also involve the connection of sources of. E. M. at a plurality of bridge points. For. example, additional: E. M. F.s may be applied at other bridge points, as. shown inFig. 20, where impedance Z2 is formed by a generator source 2' applying an E. M. F. E2 in series with an impedance Z2, which may be its internal impedance, in addition to source I connected to impedance Z1. The remainder of the circuit of Fig. 20 is similar to that of Figs. 1 or 2. It will be understood that, for a shunt-connected load Z3, a 180 phase shifter is used, although this may be omitted for a series-connected load Z3. Hence, since the transmission paths 2, 3 and l, 4 are otherwise identical, if impedances Z1 and Z3 are equal, power will be delivered from source 2' to both elements Z1 and Z3 and excluded from element Z4. If, further, the impedances Z2 and Z4 are equal, power will be delivered from source I to both elements Z2 and Z4 and excluded from element Z3. This arrangement will be referred to as a double balance.

A voltage derived from a parallel or shunt connection of a generator to the circuit may of course be impressed across any of the impedances in either a doubly balanced or any of the preceding circuits instead of a series-impressed E. M. F.

It is possible to extend balancing arrangements of the described character so that power generated. at any of the bridge points may be supplied to the two adjacent bridge points and excluded from the opposite bridge point.

In a doubly balanced system, if one of the balances is destroyed another type of interconnection of two sources is provided. For example, referring to Fig. let the balance of Z; against Z2 be destroyed so that an E. M. F. impressed on the network by source I in series with Z1 will cause currents to fiow in all of elements Z2, and Z4, but by letting the balance of Z1 against Z3 be preserved, an E. M. F. impressed on the network by source 2 in series with Z2 will cause currents in elements Z1 and Z3 but not in element Z4. Under such conditions, power from the source i may be supplied to three elements connected to the bridge, while power from source 2 is fed to only two elements and is excluded from the remaining or diametrically opposite element.

An important application of the principle of excluding power from an opposite bridge point while supplying power to bridge points adjacent the source is in a two-way repeater in a high frequency communication circuit in which a wave travelling in either direction along the line may be amplified and retransmitted in the same direction, or both directions, by means of an amplifier so connected as to prevent singing or oscillation. A repeater of the so-called 21 type, that is, a two-way repeater having a single amplifier element 22, is shown in Fig. 21 where the east line and. west line are connected to and form the impedances at two opposite bridge points of a balanced transmission line bridge of the type of Fig. 3, with amplifier 22 connected across the con jugate bridge points. Impedance adjusting networks 23 and 23 are preferably included in the east and west lines, respectively, to facilitate balancing the bridge.

In such a circuit. a wave travelling from east to west arrives at the junction of lines 2 and 3 and divides between these two paths. Due to the passage of the wave over the path including lines 2 and I an E. M. is generated at the .iunction of lines 2 and I, which, when applied to the input 24 of amplifier 22, causes a greatly amplified voltage from the output 24 of this amplifier to be applied at the junction of lines 3 and 4, from which point the amplified wave reaches the west line over two paths, i. e.. line 4 ad lines 3. 2, and i. Another portion is transmitted back to the east line over the bridge.

As is shown above, if the line impedances connected at the junctions of lines l, 4 and 2, 3 are adjusted to equality, none of the output of the amplii. or 22 is transmitted to the input at the opposite bridge point, and therefore undesirable singing or amplifier oscillation cannot occur. Similar considerations apply to the amplification of a wave arriving over the west line.

While the east and west lines of 21. are shown as shunt connected to the so far as balancing is concerned the characteristic impedances of these lines act merely as terminating impedances for the bridge, and a series connection as shown for example in Figs. 4 and 5 may be employed, in which case no transposition or phase shift is required. Such a series connection is shown in Fig. 22, of the general type of Fig. 4.

Fig. 23 represents a repeater of the so-called 22 type, that i: a two-way repeater having two separate amplifying elements 25 and 25, each serving to amplify transmission in one direction only as indicated by the arrows. In this case the east line, for example, is balanced by an impedance element Z1 at the opposite bridge point of one transmission line bridge having arms i, 2, 3, 4 to which it is connected, element Z1 having an impedance equal to the characteristic impedance of the east line. An E. M. F. is applied by awave over the east line at the bridge point between lines 2 and 3, and through these lines is applied to the output 21 of amplifier 2S and to the input 28 of amplifier 25, respectively. Due to the directional characteristics of the amplifiers, only the voltage applied to the input 28 of amplifier 25 is effective, producing an amplified voltage at the output 29 which is applied to one bridge point of the lower bridge circuit having lines I I. l2, [3, M, and thus reaches the west line.

Similarly, a wave arriving by the west line arrives at the junction between lines H and id of the lower transmission line bridge. The west line is balanced by impedance Z13 at the opposite bridge point. The incoming wave is prevented from passing to the upper bridge through amplifier 25 because of its unilateral characteristic, but is amplified by amplifier 2G and reaches the east line by way of line 3 and lines 4, I, 2 of the upper bridge. It will be apparent that energy cannot be transferred around the conducting loop formed by the two amplifiers and bridges since the input of one amplifier is applied at the conjugate bridge point of a balanced bridge from that point at which the output of the other amplifier is applied. The amplifiers therefore cannot sing around the loop.

Fig. 24 illustrates the application of the present high frequency transmission line bridge circuit to a modulator in which the carrier frequency is suppressed. In this application, a carrier source 30 supplies an E. M. F. EC through the impedance Z2 to the junction point between lines I and 2 of the bridge. The load 3| having an impedance Z4 in this case is shown for illustration as connected in series between lines 3 and 4 at their junction. A modulating source 32 is connected in series with an impedance Z1 across the junction of lines I and 4, while element Z3 is a non-linear device or mixer 33 connected across the junction of lines 2, 3. Since no transpositions are employed, no power due to the carrier source 30 will appear in load Z4 so long as the impedances Z1 of undulating sources 32 and the effective internal impedance Z: of mixer device 3| at carrier frequency are balanced. The impedance Z2 oi the carrier source and the load impedance Z4= are chosen not to fulfill the condition for balance, so that device 33- will receive power from both: the carrier and modulating sources. Due to the non-linear characteristics of this device, modulation occurs and the side bands: resulting from such modulation supply power to the load impedance Z4, thecarrier being excluded.

Fig. 25- illustrates another important applica tion of'ahigh frequency bridge circuit in providing a connection between two radio antennas either for transmitting or receiving such that there is no interaction of one antenna on theother, as for diversity reception. or directional transmission. By this means, individual tuning, phasing, or other adjustments of either antenna may be carried on without affecting the other antenna.

In Fig. 25; antenna 35' is shown as connected in the place of impedance Z2 of the basic bridge circuit of Fig; 1, whileantenna 36 is shown as connected in the position of impedance Z4. These designations will be retained for the i'mpeda-nces ofthe respective antennae. A device 31, 31',

which may be either a transmitter or receiver depending on whether antennae 35 and 36 transmit orreceive energy, is shown connected in the position of Z1, while impedance element Z3 balances the impedance of the said transmitter 3'!- or receiver 31 originating in either antenna will have no effect on the other antenna when the bridge is balanced, but the antennae 35, 36 will supply energy to I'vceiver 31 during reception, while on the other hand; during transmission the antennae 35, 36 will beindependently supplied by transmitter 31-.

Fig. 26 shows another form of modulating circuit employing a transmission line bridge, dif fering from Fig; 24 only in the manner in which the modulating E. M. F. is supplied. In this circuit impedance Z1 of the basic circuit is the output impedance of an electron discharge device 38 which, by the inclusion of a, suitable biasing E: M. F; in the input circuit, as from battery 36, may be varied under the control of an E; M. F: supplied by modulating source 39. If, for a par-- ticular value of tube output impedance, a balance against impedance Z3 is obtained, no energyfrom carrier source 39' is supplied to load 38. As the output impedance of tube is caused to vary from this value'by the input modulating voltage supplied from modulating source 39; the carrier energy supplied toload 3| varies proportionally, so that modulation takes place.

has been pointed out, my invention includes circuits in which there are substituted for transmission lines with distributed constants, simulated lines comprised of lumped impedances. Thus, in any of the embodiments of the inventi'on described above. the indicated transmission line. can be replaced by an artificial line having lumped-impedance circuit elements and simulating in well-known manner a distributedconstant transmission line. A generalized bridge circuit using such artificial lines is illustrated in In general, to simulate actual. transm ssion lines, artificial. lines will, have series resistance and inductance and shunt capacitance and. con.- ductance, although the relative importance of these several factors will vary With the frequency range for which the apparatus. is designed, and one or more.v of them may be omitted in some. applications. As an illustration, in. the circuit. of Figs 27; only series inductance L and shunt capachance C. are shown.

It will be apparent that E. M. F;s

Accordingly; there has. been. described a basic transmission line bridge and its variations, use fill with manydi'iferent types oftransmission lines and in many different types of systems; Such apparatus is especially useful with waveguides and coaxial transmission lines, thereby permitting use at microwave frequencies. Itwill be understood that any type of transmission line may be used in any of the above systems, in accordance with the present teachings.

Also, in view of the well known Reciprocity iheorem, if all circuit elements are linear and bilateral, the source or generator and the indicator or load are'interchangeable. Also, either maybe series-connected" or shunt-connected, in accordance with theprinciples set out above.

In somecases the impressed E. M. F., instead of being supplied by an actual generator, may be due to a disturbance induced in one of the lines either externally or byinternal variations of-the line or terminating impedances, and it maybe desired to. prevent any effect of this disturbance from reaching the opposite bridge point, while that point is receiving power fromadjacent bridge points. The above-described balanced bridge circuits: provide means: for: accomplishing this result.

As many changes could be made in. the above. construction and many apparently widely different embodiments of this invention could; be made without departing from the scope thereof, it: is intended that. all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a-limiting sense.

What is claimed is:

1'. A carrier suppression modulator comprising a bridge network having transmission lines as the four arms thereof, a source of carrier frequency alternating current connected atv one bridge point of said circuit, a series connected load to which side-band power is supplied connected at the" opposite bridge point, a source of modulation frequency alternating current connect'ed at one of the conjugate bridge points: and

a non-linear device connected at the other conjugate bridge point, said source. of carrier frequency alternating current, said series connected load; said source ofmodulation frequency al ternating current, and said non-linear device each possessing internal impedance, and said modulating frequency alternating current source and non-linear device having effectivelyequal impedance at carrier frequency whereby carrier frequency power is substantially excluded from said load.

2. A modulator'circuit comprising a bridgcnetwork having transmission linesas the four arms thereof; a source of carrier frequency alternating current connected to said bridge atone bridge point thereof, a series connected. load connected at the opposite bridge point, said sourceof carrier frequency alternating current and said series connected load each possessing internal impedance, a constant impedance connected at one oi the bridge: points, conjugate to said first two points, a variable impedance device. connected at the other conjugate bridge point, and means. for varying the, impedance of said variable impedance device toeflect modulation of they power supplied to said. load by said source of carrier frequency.

3.. Amodulator circuit as claimed. in claim 2, in whichysaid variable impedance'device is an electron discharge device whose output imped- 15 ance is controlled by said impedance varying means.

4. A bridge network having transmission lines as the four arms thereof, a source of energy at one bridge point of said circuit, a series connected load at the opposite bridge point, a control source of energy connected at one of the conjugate bridge points and a balancing impedance at the other conjugate bridge point, said control source and said balancing impedance having substantially equal impedances, whereby said load is controlled independently of said energy source.

5. Bridge circuit means having transmission lines connected as the arms thereof with opposite bridge points and conjugate bridge points formed at the junctions of the arms, energy supply means connected to one of said opposite bridge points, a control source connected to one of said bridge points, and a load series connected to one of the remaining bridge points, whereby the load is controlled by the control source independently of the energy supply means.

6. A circuit network comprising four transmission lines and four impedance elements, said transmission lines being connected in tandem as a continuous loop with one of said impedance elements connected at the junction of each pair of adjacent lines, thereby forming a bridge circuit with said impedance elements at the four bridge points thereof, at least one of said impedance elements being series connected between adjacent lines, an alternating current source connected to impress a potential upon one of said impedance elements for initiating the transmission of electromagnetic waves in opposite directions along a pair of adjacent lines and the lines respectively succeeding them, and a control source connected to impress a potential upon a difierent one of said impedance elements at one of the bridge points adjacent to said alternating current source.

7. A bridge type network comprising four transmission lines connected as the arms of the bridge, impedance elements connected between adjacent arms at each of the four bridge points, a high frequency alternating current source connected to impress a potential across one of said impedance elements for initiating the transmission of electromagnetic waves over the two adjacent arms of the bridge, means actuated by the potential across the impedance element opposite said source excited element, said means being connected in series between adjacent lines, and a control source connected to impress a potential upon a different one of said impedance elements at one of the bridge points adjacent to said alternating current source.

8. High-frequency apparatus comprising a pair of wave-guide paths, an input connection coupled to both said paths, an output connection also coupled to both said paths, one of said connections being series-connected to said waveguide paths and the other connection being shunt-connected to said two paths, a transmitter coupled to one of said connections, a receiver coupled to the other of said connections, and a pair of circuits also coupled to said wave-guide paths, one of said circuits comprising a radiating means and the other comprising a circuit element offering an impedance value adapted to maintain a slight degree of bridge unbalance between said transmitter and said receiver.

9. High-freouency apparatus comprising a bridge circuit having a pair of enclosed-field transmission line means, an input connection coupled to both said transmission line means, an output connection also coupled to both said transmission line means, one of said connections being series-connected to said two transmission line means and the other connection being shunt-connected to said two transmission line means, a transmitter coupled to one of said connections, a receiver coupled to the other of said connections, and a pair of circuits also coupled to said two transmission line means and determining by their impedance values the transfer of energy between said connections, one of said circuits comprising antenna means adapted to transmit energy from said transmitter and also adapted to receive energy to be supplied to said receiver and the other of said circuits comprising balancing impedance element means.

10. Apparatus as in claim 9 wherein said impedance element means is adjusted to permit a slight amount of energy from said transmitter to reach said receiver, said slight amount of energy thereby serving as a local oscillation source for superhetercdyne reception.

11. A station for high-frequency communication comprising a bridge circuit having a pair of wave-guide elements, a first connection coupled to both said elements, a second connection coupled also to both said elements, one of said connections being series-connected to both said wave-guide elements and the other connection being shunt connected to both said wave-guide elements, a source of high frequency oscillations coupled to one of said connections, a receiver for high frequency oscillations coupled to the other of said connections, and antenna means also coupled to one of said wave-guide elements and adapted to radiate energy produced by said source of high frequency oscillations and to receive energy for supply to said receiver.

12. Apparatus as in claim 11 wherein said receiver is of the superheterodyne type having a mixer stage and wherein balancing impedance element means is provided coupled to one of said two wave-guide elements opposite said antenna coupled means and adjusted to provide a small degree of unbalance for said bridge circuit whereby said mixer stage is excited by energy received by said antenna means and by a small amount of energy derived directly from said source of high frequency oscillations and serving as local oscillation energy for actuating said superheterodyn receiver.

13. A station for high-frequency communication comprising a bridge circuit having a pair of transmission-line elements, a first coupling series-connected to both said transmission-line elements, a second coupling shunt-connected to both said transmission-line elements, a high-frequency source coupled to one of said couplings, a high-frequency receiver coupled to the other of said couplings, and an antenna also coupled to one of said transmission-line elements.

WILMER L. BARROW.

References Cited in the file of this patent UNITED STATES PATENTS Number 

